Enterprise service bus — ESB, is an architectural pattern that centralized software component performs integrations between applications.

It performs transformations of data models, handles connectivity and messaging, performs routing, converts communication protocols and potentially manages the composition of multiple requests.

The ESB can make these integrations and transformations available as a service interface for reuse by new applications. In this model, service consumers and service providers collaborated via middleware messaging components, often referred to as an Enterprise Service Bus, or ESB.

Business logic would be built into the ESB to integrate providers and consumers. Service consumers could then find and communicate with these providers using the ESB.

The core of SOA success depends on an Enterprise Service Bus (ESB) that supports dynamic alignment of business process interactions.

Service Bus Component of SOA

ESBs have been evolution of integrated middleware infrastructure technology by combining features from previous technologies with new services, such as message validation, transformation, content-based routing, security and load balancing.

https://docs.oracle.com/cd/E13171_01/alsb/docs30/concepts/introduction.html

ESB provides an efficient way to build and deploy enterprise SOA. ESB is a concept that has gained the attention of architects and developers, as it provides an effective approach to solving common SOA problems about service orchestration, application data synchronization, and business activity monitoring.

Despite the promises of SOA, implementing this approach often increased complexity and introduced bottlenecks. ESB can be bottlenecks and single point of failure on SOA architectures.

Maintenance costs became high and ESB middleware expensive. Services tended to be large. They often shared dependencies and data storage. In the end, SOAs often resulted in a ‘Distributed Monolithic’ structure with centralized services that were resistant to change.